1. Field of Technology
The present invention relates to a magnetic ink character reading apparatus that acquires magnetic ink character data by reading magnetic ink characters or other magnetic pattern, and relates more particularly to improving magnetic noise cancellation performance.
2. Description of Related Art
Check processing machines that read magnetic ink recognition characters (MICR characters) printed on checks are commonly used in financial institutions and other businesses. Check processing machines with a media transportation mechanism typically use a stepping motor to convey the checks past a magnetic head to recognize the magnetic ink characters. Magnetic noise cancellation is essential in such machines in order to achieve a high character recognition rate.
A major source of magnetic noise is the stepping motor used in the transportation mechanism. Addressing this problem, Japanese Unexamined Patent Appl. Pub. H9-282410 teaches a method for canceling magnetic noise. More specifically, this method first acquires magnetic signals detected when MICR characters are not detected as magnetic noise synchronized to the drive pulse through multiple cycles and determines the median as a data correction value. When reading the magnetic ink characters, this correction value is then subtracted from the magnetic detection signal, thereby removing the magnetic noise from the MICR detection signal.
The transportation mechanism also generally has a mechanism for detecting double feeding, that is, two or more overlapping checks being conveyed together. Japanese Unexamined Patent Appl. Pub. S60-144256 teaches technology for monitoring double feeding by using a lever that is pushed into the transportation path by a spring. When a check is conveyed and passes under the lever, the lever is raised according to the thickness of the check. This displacement of the lever is detected by a photodetector to measure the check thickness and thereby detect double feeding.
Checks read by the foregoing check processing machines may be wrinkled or soiled, and the magnetic detection sensitivity of the foregoing check processing machines is therefore preferably increased in order to obtain a sufficiently high character recognition rate even with checks in poor condition, but magnetic noise from weak sources other than the stepping motor becomes a problem as the sensitivity of the magnetic head rises. Furthermore, as the size of such check processing machines has been reduced, it has become increasingly difficult to assure sufficient space between the magnetic head and other electronic components, thus exacerbating problems related to magnetic noise.
FIG. 11A is a waveform diagram showing an example of the magnetic detection: signal before processing to remove magnetic noise. FIG. 11B shows the waveform of the signal shown in FIG. 11A after removing magnetic noise from the stepping motor. As will be known from these figures, magnetic noise remains even after magnetic noise from the stepping motor has been removed.
The primary component of this residual magnetic noise has a cycle with a period different from the control cycle of the stepping motor, and our research has shown that the magnetic head picks up magnetism from the drive current of the LED used in the photodetector of the double feed detection mechanism.
At least one embodiment of the present invention is thus directed to improving the magnetic noise cancellation performance and increasing the character recognition rate in a magnetic ink character reading apparatus having a media transportation mechanism that conveys a recording medium by means of a stepping motor, and a paper thickness detection mechanism for detecting paper thickness by means of a photodetector in the media transportation mechanism.